1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus using the same. More particularly, the present invention relates to a device suitably used for an image forming apparatus, such as a laser beam printer or a digital copying machine having an electrophotographic process, in which at least one light beam emitted from a light source means is deflected by a deflection element and then optically scans a scanning surface through an image forming element having an fθ characteristic, thereby recording image information.
2. Related Background Art
In a conventional scanning optical apparatus used for a laser beam printer (LBP), a digital copying machine, or the like, the light beam optically modulated according to an image signal and emitted from a light source means is regularly deflected by a light deflector composed of a rotary polygon mirror (polygon mirror). The deflected light beam is converged to form a spot on a surface of a photosensitive recording medium (photosensitive drum) by a scanning optical element having an fθ characteristic, and optically scans the surface of the photosensitive drum to record an image thereon.
FIG. 5 is a schematic diagram that shows the main part of a conventional scanning optical apparatus of this type.
In this figure, diverging light beam emitted from a light source means 51 is converted into a substantially parallel light beam by a collimator lens 52, an aperture stop 53 limits the parallel light beam (quantity of light), and the limited parallel light beam strikes a cylinder lens (cylindrical lens) 54 having a predetermined refracting power only in the sub-scanning direction. The substantially parallel light beam striking the cylinder lens 54 is emitted as it is (in the state of substantially parallel rays) in a main scanning cross-section. In a sub-scanning cross-section, the light beam is converged to form a nearly linear image on a deflection surface (a reflection surface) 55a of a light deflector 55 composed of a rotary polygon mirror (polygon mirror).
The light beam reflected and deflected by the deflection surface 55a of the light deflector 55 is guided by an image formation means (fθ lens) 56 having an fθ characteristic to strike a photosensitive drum surface 58 to be scanned. By having the light deflector 55 rotate in the direction of an arrow A, the photosensitive drum surface 58 is optically scanned in the direction of an arrow B. In this manner, an image is recorded on the photosensitive drum surface 58 that is the recording medium.
In recent years, various scanning optical apparatuses (multi-beam scanning optical apparatuses) have been proposed in response to the increasing demand for high-speed and high-resolution LBPs. Each of such scanning optical apparatuses adopts a multi-beam laser that uses a plurality of laser light sources (light-emitting units) as its light source. With this construction, the scanning optical apparatus simultaneously scans surfaces of a plurality of recording media by irradiating the surfaces with a plurality of light beams.
In addition to high-speed and high-resolution image forming apparatuses having the electrophotographic process, various color image forming apparatuses that support high-speed color image formation have also been proposed. FIG. 6 is a schematic diagram that shows the main part of a tandem type color image forming apparatus that simultaneously uses a plurality of scanning optical apparatuses described above to record image information in various colors on the surfaces of a plurality of different photosensitive drums.
In the color image forming apparatus shown in this drawing, four scanning optical apparatuses (61, 62, 63, and 64) having the construction shown in FIG. 5 are arranged so as to respectively correspond to four colors: C (cyan), M (magenta), Y (yellow), and B (black). These scanning optical apparatuses record image signals on the surfaces of photosensitive drums (71, 72, 73, and 74) in parallel. With this construction, the color image forming apparatus prints color images at high speed.
One crucial aspect of the scanning optical apparatuses used for such a color image forming apparatus is the productivity and cost. Therefore, in usual cases, the scanning optical apparatuses are provided with image formation means (fθ lenses) that have been produced at low cost by molding of plastic.
In the scanning optical apparatus that uses a plastic lens, deviations in an image forming position are caused by changes in a focus position and chromatic aberration of magnification that are caused by various factors. Three major factors are given below.
Deviation in initial wavelength between a plurality of laser light sources
Deviation in wavelength caused by mode hopping of a semiconductor laser due to environmental variations
Fluctuation in refractive index of the plastic lens due to environmental variations
The deviations in the focus position result in the enlargements of a beam spot and degrade image quality. Deviations in the image forming position due to chromatic aberration of magnification cause magnification changes of recorded images. In particular, in a tandem type color image forming apparatus, the differences in magnification change among a plurality of scanning optical apparatuses cause registration deviations of each color (chromatic deviations), which results in degraded image quality.
A color image forming apparatus that solves this problem is proposed, for instance, in Japanese Patent Application Laid-open No. 11-223784. This color image forming apparatus corrects chromatic aberration of magnification and compensates for the temperature at a focus position using an image formation means constructed by combining a refractive optical element with a diffraction optical element.
The following is a description of an ordinary color image forming apparatus that includes an image formation means constructed by combining a refractive optical element with a diffraction optical element. (a-1) The power of the diffraction optical element which is disposed at a far side with respect to the optical deflector needs to be increased to reduce the magnification (the sub-scanning magnification) of the image formation means in the sub-scanning direction as necessary. It is preferred to set the power ratio between the refractive optical element and the diffraction optical element at a desired ratio for an aberration correction purpose. It is difficult, however, to produce a high-power diffraction optical element because a grating pitch of its diffraction grating is reduced according to the increase in the power of the diffraction optical element. When the sub-scanning magnification is large, the scanning line deviates from a desired position due to various factors, such as the errors caused during the processing of deflection surfaces of a polygon mirror and the vibrations of the apparatus itself. This tends to cause jitter, which is a critical design factor because jitter causes registration deviations of each color (chromatic deviations) in a color image forming apparatus.
(a-2) When a scanning optical apparatus is manufactured and assembled using a plurality of optical elements (a refractive optical element and a diffraction optical element, in this case), if the refractive optical element and the diffraction optical element are eccentrically arranged due to manufacturing errors, the traveling path of a laser spot (so-called scanning line) is bent in the scanning optical apparatus. For instance, in the conventional case shown in FIG. 5, if a toric lens 56a is arranged at a position that is shifted 0.1 mm in a direction perpendicular to the plane of paper at normal mechanical accuracy, the bend degree of the scanning line becomes about 29 μm. In particular, in a tandem type color image forming apparatus, the differences in bend degree of scanning line among scanning optical apparatuses corresponding to respective colors cause registration deviations of each color (chromatic deviations). As a result, image quality is degraded.